Because of its speed, thin layer chromatography (TLC) is a widely used analytical method. It has been possible to increase its efficiency constantly by continuous further developments and improvement. With the HPTLC technique (high performance thin layer chromatography), it has been possible to achieve results which largely are analogous to those of high pressure liquid chromatography (HPLC). It is thus a rapid and inexpensive alternative to the more time-consuming HPLC analysis, which required more expenditure of equipment. In the end, the advances in the TLC/HPTLC technique are based on improved porous silica gels, which are chiefly used as adsorbents and carrier materials in thin layer chromatography.
In particular, the introduction of silica gel surfaces chemically modified with organic groups (for example, as described in Journal High Resol. Chromatogr. Comm. 3; 215-240, 1980) led to a considerably wider range of applications. It thereby became possible to react the Si-OH groups of the silica gel, which are polar by nature, with suitable lipophilic or partially lipophilic organic molecules in a chemical reaction, i.e., to convert them into hydrophobic (reversed phase) or partially hydrophobic groups.
A further enrichment of the TLC technique is in situ modification, that is to say chemical modification on a finished layer of silica gel, for example, as has been described in German Patent Specification Nos. 2,712,113 or 2,809,137. It is thereby possible to produce, inter alia, homogeneous packing and coating surface structures and purer layers.
In the known preparation processes for chemically modified silica gel, the particular degree of covering of the modified adsorbent with organic groups is predominantly determined by the nature of the groups in the modifying reagent which are capable of undergoing reaction with the Si-OH groups of the silica gel skeleton. To obtain the highest possible degrees of conversion, correspondingly substituted reactive halogenosilanes are usually employed as the modifying reagent. The use of alkoxysilanes which are in themselves slow to react can evidently, according to German Patent Specification No. 2,426,306, lead to quite high degrees of covering of the silica gel surface if catalysts are simultaneously present during the silanization. Although the high degrees of conversion guarantee a good interaction between the hydrophobic carrier matrix and the hydrophobic substance, they mean that the hydrophobically modified layer of silica gel can no longer be wetted by eluting agents and spray reagents containing a large amount of water.
In order to ensure the required wettability with water, the hydrophobic character of the modified layer of silica gel must be reduced. Using less than the stoichiometric amount of halogenosilanes or mixing silica gel with a maximum degree of modification and unmodified silica gel before the coating operation sometimes gives rise to considerable disadvantages, as has been shown in practice. The method usually practiced in which the hydrophobic character of silica gel layers to be modified in situ is reduced by using alkoxy- or aralkoxy-silanes which are slow to react for the chemical modification, frequently has the disadvantage that the degree of covering is greatly reduced. The actual reversed phase effect therefore is frequently only inadequate.
In the case of chemical modification of silica gel layers with organic molecules containing polarhydrophilic groups, such as, for example, alkyl chains carrying epoxy or amino groups, it is known that exclusively, the alkoxy- or aralkoxy-silanes which are slow to react can be employed for the modification since the corresponding halogenosilanes do not exist. Silica gel layers modified in this manner accordingly have degrees of covering which are in principle lower, and in some cases too low. A higher surface concentration would be advantageous with this type of modification, however, because, in contrast to the purely hydrophobically modified layers, the layer in any case has the desired wettability with water, even with maximum reaction, because of the groups which have been introduced, some of which are quite polar.